It is known that motor vehicle bumpers have cross-beams whose function is to absorb a large portion of the energy from impacts.
A test in wide use, in particular in the United States, for verifying the strength of a bumper is known as the “pole test”. Such a test consists in subjecting the rear of the vehicle to an impact on its middle axis at a speed of 8 kilometers per hour (km/h) against a fixed pole that is 7 inches (about 178 millimeters (mm)) in diameter. The test is considered to be satisfactory if the cost of repairing the vehicle after such a test is lower than a certain threshold.
In order to limit the repair costs resulting from such an impact, it is essential to minimize the damage to the rear of the vehicle, and thus to limit the extent to which the pole penetrates into the rear face of the vehicle during the impact, e.g. by putting into place a cross-member that is sufficiently strong to reduce the extent to which impacting objects penetrate in the event that an impact occurs.
In addition to the pole test, the Danner test requires motor vehicles to anticipate considerably the transfer of forces onto energy absorbers during an impact. In order to absorb such energy, it is possible to mount extensions between the side rails of the vehicle and the bumper beam.
Metal extensions or cross-members have long been used in bumpers because of their good rigidity. However, such parts suffer from the drawback of being relatively heavy. In addition, they are not easy to adapt to particular geometrical configurations that might be required by the vehicle, sometimes requiring additional parts that can be referred to as “absorbers”.
That is why attempts have been made to replace the metal extensions or beams with parts that are made of plastics material and that satisfy better the two criteria of weight and of geometrical shape.
However, because of the low rigidity inherent to the materials used, such parts made of plastics material must have complex structures, in particular ribs, and the material used must be mixed with reinforcing fibers which increase its cost and make manufacturing the parts more complex, thereby tending to diminish the advantages procured by use of plastics material, without completely eliminating them.